1. Field of the Invention
The present invention relates to a differential interference contrast microscope for use in investigating transparent biological substances or specimens such as cells and bacteria as well as a fine pattern of projections and depressions formed on a surface of a semiconductor substrate, e.g. a silicon wafer. The present invention also relates to a microscopic image processing system using such a differential interference contrast microscope.
2. Related Art Statement
FIG. 1 shows a known differential interference contrast microscope of transmission type. The microscope comprises, in addition to illumination light source 1, condenser lens 2 and objective lens 3 which are generally provided in a conventional optical microscope, polarizer 5 and Nomarski prism 6 which are arranged in this order between the illumination light source 1 and the condenser lens 2, and Nomarski prism 7 and analyzer 8 which are arranged in this order between the objective lens 3 and an imaging plane 4. In such a differential interference contrast microscope, after a light ray emitted by the light source 1 is converted by the polarizer 5 into a linearly polarized light ray, the light ray is divided by the Nomarski prism 6 into ordinary and extraordinary light rays. Then, these two linearly polarized light rays are projected by the condenser lens 2 onto a specimen or object 9 under inspection. The ordinary and extraordinary light rays transmitted through the specimen 9 are combined via the objective lens 3 on a same light path by means of the Nomarski prism 7. Then, the combined light rays are made incident upon the analyzer 8 to form a differential interference contrast image on the imaging plane 4.
Here, an amount of the separation between the ordinary light ray and the extraordinary light ray on the specimen 9 under inspection is called an amount of wavefront shear or a shear amount of wavefront. It has been known that an amount of wavefront shear is an important parameter for defining the contrast of the differential interference contract image and the resolving power of the microscope. For instance, in Japanese Patent Laid-open Publication Kokai Hei 7-35982, there is described that in order to obtain a practical contrast under the inspection with naked eye, it is necessary to increase an amount of wavefront shear to a certain extent. However, when an amount of wavefront shear is increased beyond the resolving power of the objective lens of the microscope, a so-called double image is inspected to decrease the resolution of the image.
Therefore, in known differential interference contrast microscope, in order to investigate various objects, the shear amount of wavefront between the two linearly polarized light rays on an object under inspection is usually determined such that the contrast and the resolution of the image inspected by the naked eye are balanced suitably.
However, there has been proposed to change an amount of wavefront shear in accordance with objects under inspection. For instance, in a microscopic image processing system using the transmission type differential interference contrast microscope, a differential interference contrast image obtained by the microscope is picked-up by an electronic image sensing device and a contrast of the image is enhanced by an image processing method. In such a contrast enhancing method, it is possible to monitor clearly a differential interference contrast image having a too low contrast to be seen by the usual inspection with the naked eye. In order to further increase the resolution of the monitored image, an amount of wavefront shear has to be decreased below a conventional value which has been used for inspecting the differential interference contrast image with the naked eye as described in the above mentioned Japanese Patent Publication Kokai Hei 7-35982.
A reflection type differential interference contrast microscope has been also used to inspect a fine structure such as a gap portion of a magnetic head. Also in this case, it has been known that the differential interference contrast microscopic image can be inspected much more clearly by decreasing an amount of wavefront shear.
As explained above, recently it has been desired to obtain a differential interference contrast image by changing an amount of wavefront shear and differential interference contrast microscopes which can offer different amounts of wavefront shear have been available from almost microscope makers. A similar faculty for changing an amount of wavefront shear has been also required in a microscopic image processing system using a differential interference contrast microscope such as various measuring equipments. For instance, not only the above mentioned microscopic image processing system using the contrast enhancing method, but also a step measuring device for inspecting a step of an object under inspection by utilizing the differential interference contrast, a phase difference measuring device for measuring a phase distribution of a transparent object under inspection by utilizing the differential interference contrast and a position detecting device for detection a position of an alignment mark on a semiconductor wafer have encountered the same problem. In these devices, the measurement and position detection could be performed much more precisely by applying a suitable amount of wavefront shear for objects under inspection.
However, in known differential interference contrast microscopes, the ordinary and extraordinary light rays are obtained by using the Nomarski prism which is made of a birefringent crystal, and therefore it is necessary to prepare a plurality of Nomarski prisms which are designed to provide different wavelength shears. It should be noted that since the Nomarski prism is manufactured by precisely processing the birefringent crystal, it is liable to be rather expensive. Therefore, a cost for preparing a plurality of expensive Nomarski prisms becomes very high. Moreover, even if a plurality of Nomarski prisms are prepared, each of the prisms has their own specific shear amounts of wavefront, an inspection could not be always possible with an optimum amount of wavefront shear.
In "OPTICA ATCA", 1972, vol. 19, no. 12, 1015-1026, M. Pluta has reported a differential interference contrast microscope with a variable amount of wavefront shear. In this known differential interference contrast microscope, first and second sets of 1/2 wavelength plate and Nomarski prism are provided on the objective lens side and condenser lens side, respectively, and in each of the first and second sets, the 1/2 wavelength plate and Nomarski prism are arranged rotatable about an optical axis to change an amount of wavefront shear.
However, in this known differential interference contrast microscope, there is a problem that a positional shift of image might occur upon the rotation of the Nomarski prisms when surfaces of the Nomarski prisms are not in parallel with each other. Moreover, this microscope requires four expensive Nomarski prisms instead of two Nomarski prisms in the conventional differential interference contrast microscope, and thus a cost is apparently increased.